This group investigates the mechanisms that govern the intracellular localization and trafficking of newly synthesized proteins in the secretory pathway. Studies focus mainly on two problems: (a) the assembly of multi-subunit complexes in the endoplasmic reticulum (ER) and (b) the mechanisms of protein localization to the trans-Golgi network (TGN). Studies of protein assembly in the ER were done using as a model the multi-subunit class II molecules of the major histocompatibility complex (MHC). Class II MHC molecules are composed of two polymorphic chains (alpha and beta) and a protein known as the invariant chain (I). Our previous studies had shown that various incompletely assembled forms of class II MHC molecules are largely retained within the ER, where they form large aggregates bound to the ER chaperone, BiP. More recently, we have found that similar aggregates are not merely aberrant products of incomplete assembly, but act as true intermediates in the assembly process. Analyses of the formation and dissociation of such aggregates and of the interaction of class II species with ER chaperones have provided insights into the sequential steps of the assembly process Our previous work on the assembly of class II MHC molecules had identified a potential glycine strip motif within the transmembrane domains of the alpha and beta chains that was required for efficient assembly in the ER. We have recently found a similar motif within the HIV-1 fusion peptide. Mutagenesis analyses of this motif show that it plays a critical role in syncytia formation and infectivity. We have also continued our studies on protein localization and sorting within the TGN. The cytoplasmic tail of the mammalian endopeptidase, furin, was found to contain information that is both necessary and sufficient for localization to the TGN. A tyrosine-based motif (YKGL) was shown to be partially involved in signaling a TGN localization. A related tyrosine based motif (YTPL) was found to target the MHC-like molecule, Mb, to a novel endosomal compartment. These findings suggest the existence of intracellular sorting mechanisms that recognize cytoplasmic tyrosine-based motifs with distinct specificity.